Your BMW warranty starts on the date of sale of your new car. If you buy your car but choose not to drive it home yet, your warranty will still start on the day you officially purchase your vehicle.
In a lot of ways, this is very helpful, as it means that you can reference your sale documents to see how much longer your warranty will stay in place. Remember that BMW standard warranties last for four years or 50,000 miles, whichever comes first, so your warranty will expire four years from the day you buy your car.